A particularly important application of the invention relates to hydraulic assistance apparatus mounted on road vehicles such as heavy goods vehicles that can travel at some speed (over 50 kilometers per hour (km/h)). For such an application, the hydraulic assistance apparatus must satisfy certain additional constraints: firstly, it must guarantee full safety when the vehicle is traveling at high speeds, i.e. it must be almost impossible for the assistance motors to be triggered in untimely manner; secondly the apparatus must make it possible for the assistance motors to be switched on while the vehicle is advancing, and not merely when it is at a standstill or while it is traveling at very low speed (less than 5 km/h).
In such assistance apparatus, in known manner, a hydraulic pump having a variable delivery rate is used as the main pump. Such a pump makes it possible to adapt the speed of the motors as a function of need.
The invention relates in particular to hydraulic assistance apparatus including such a pump as the main pump, and more generally such apparatus in which the main pump is a pump that can be actuated without delivering, but that then requires some minimum pressure to be applied to its main orifices. If such pumps are actuated while pressure lower than the above-mentioned minimum pressure prevails in either of the main orifices of the pump, a risk of damaging the pump ensues.
However, it should be noted that the invention does not relate to hydraulic transmission apparatus equipping vehicles in which the main transmission is itself hydraulic, and therefore includes a main pump actuated substantially continuously while the vehicle is operating.
In assistance apparatus such as the apparatus to which the invention relates, since the assistance apparatus is used only occasionally, the problem arises of activating the hydraulic transmission apparatus.
The solution usually chosen consists in providing a clutch system, interposed between the internal combustion engine of the vehicle or of the machine and the main pump of the apparatus. Unfortunately, such a clutch system is quite costly, occupies a large volume, and requires a considerable amount of maintenance.
For hydraulic assistance systems, in addition to providing a clutch system, it is necessary to provide hydraulic motors having radial pistons of the positively declutchable type, thereby making it possible to avoid the motors rotating unloaded. Such a motor has a cylinder block in which cylinders are arranged for slidably receiving the pistons. Such a motor is suitable for taking up a work configuration, in which the pistons can generate drive torque or braking torque under the effect of a fluid pressure difference between the motor ducts, and a “freewheel” configuration, in which the pistons are maintained in a retracted position inside the cylinders and do not generate any torque. The pistons are maintained in the retracted position under the effect of mechanical means such as springs or the like, and/or under the effect of a pressure prevailing in a space internal to the motor that is arranged, in general, inside the casing that contains the cylinder block.
In such hydraulic transmission apparatus including such motors, it is useful at all times to have at least some minimum pressure that can be applied to the insides of the internal spaces of the motors and thus that can maintain said motors in the positively declutched positions, with their pistons retracted into their cylinders.
The improvement adopted in such circuits consists in making provision for the main pump to be declutchable by means of a clutch, but conversely for the internal combustion engine to actuate the auxiliary pump or booster pump of the apparatus continuously. Said auxiliary pump or booster pump then guarantees that some minimum pressure is delivered continuously, which pressure can be used to keep the motors safe in the positively declutched state.
Since the main pump is declutched when no assistance is requested, this guarantees that there is no risk of it being damaged under the effect of insufficient pressure at its main orifices.
That improvement still suffers from the drawback of using a clutch, but it does procure a certain amount of safety for use of the described positively declutchable hydraulic motors. However, the auxiliary pump being driven continuously gives rise to non-negligible power consumption. In addition, the internal combustion engine has to have two outlets, one towards the clutch connected to the main pump, and the other towards the auxiliary pump. This gives rise to compactness problems, and requires the internal combustion engine to have a specific design.